diff --git a/web/src/components/ActivityCalendar/useCalendarMatrix.ts b/web/src/components/ActivityCalendar/useCalendarMatrix.ts
index 0fae3651c..0c8dab832 100644
--- a/web/src/components/ActivityCalendar/useCalendarMatrix.ts
+++ b/web/src/components/ActivityCalendar/useCalendarMatrix.ts
@@ -30,7 +30,7 @@ const createCalendarDayCell = (
     isCurrentMonth,
     isToday: isoDate === today,
     isSelected: isoDate === selectedDate,
-    isWeekend: WEEKEND_DAYS.includes(current.day()),
+    isWeekend: WEEKEND_DAYS.includes(current.day() as 0 | 6),
   };
 };